J. Smith at Baylor has shown that when poly(A)+ RNA derived from human diploid fibroblasts at late passage is microinjected into the same cell type at early passage, the growth of the early passage cells is halted. A functionally identical activity derived from rat liver (by K. McClung and J. Smith) has a single molecular weight peak of activity on sucrose gradients. These data suggest that a single messenger RNA has the ability to shut off cell growth, and dilution experiments suggest that the message is abundant (1/100-1/1000 of total message). We are attempting to clone this mRNA by two different methods: 1) hybrid selection screening of a cDNA library, and 2) construction of a cDNA library in a T7 vector that should permit us to make synthetic mRNAs that can be tested directly by microinjection.